Flash photography in Av mode

Photography's been a hobby of mine for many years. Lately i decided to become a professional. Naturally, the first thing i did was buy a boatload of gear.

Up until now i've never used or owned a proper flash - but now i do, and i need to learn how to use it properly. One of the things that i can't figure out came up last night - i was shooting a wedding(brother-in-law, a hired professional was there as well) and as it got darker, i realized my flash wasn't giving me the exposures i was hoping for.

To explain - the built-in flash on my Minolta used to alter the shutter speeds and bump them in to the 60-90 area, if i chose to pop it. However, the 580exII on my 30D doesn't do that. The exposure stays similar to what i would use had i not had the flash.
How come? If i wanted a f2.8 at 1/15th i'd put the flash back in the bag.
Also, i noticed that if i put the camera in P mode, the shutter jumps up to the familiar 60-90.

What gives?

I guess what i really need is a good book/website on flash photography.
Thanks.

On the Rebel XTi, the camera will normally attempt to use a slow shutter speed, down to 1/30, so that the background is properly exposed as well as the foreground in the flash. This is called slow shutter speed sync. The 30D is probably similar. You can disable that function via a custom menu option so that the camera always uses 1/200 instead of attempting a slow speed while in Av mode.

You didn't say if the exposure problem was under or over exposure of the flash illuminated area. If so, you can also try flash exposure compensation, via a menu item.

You didn't say if the exposure problem was under or over exposure of the flash illuminated area. If so, you can also try flash exposure compensation, via a menu item.
Best...

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Actually, the exposures were right on the money, from what i saw on the LCD(haven't had a chance to look at the pics yet). The problem was with blur, i think.
Again, i haven't gone through the ~300 pics that i took last night, so i don't really know what the results are.

Just seemed odd, from my limited flash experience, that even with the flash on, the shutter speed was so slow. I guess otherwise the background would be black.
I'll take a look at that custom option,thanks!

Anyone have a website that explains all the basics, and beyond, of flash photography?

Anyone have a website that explains all the basics, and beyond, of flash photography?

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I have just the one for you. It's often called the EOS Flash Bible. It explains how Canon has programed the flash to act differently when in Av than when in P or M. Basically, when you use Av...the camera & flash are in 'fill' mode and it tries to expose for the ambient light and only gives you enough flash for fill.

Usually when I'm using flash in dark situations, I put the camera into manual mode. The flash will put out enough power for the chosen aperture (provided you are within range)...and you set the shutter speed to vary the amount of ambient exposure. Then I'll adjust the FEC to fine tune the exposure from the flash.